This patent specification relates to semiconductor lasers. More particularly, it relates to vertical cavity surface emitting lasers (VCSELs) in which electric current requires lateral confinement in passing through an active region.
Solid-state semiconductor lasers represent desirable light sources for a variety of applications including optical data communications, telecommunications, and other applications. A vertical cavity surface emitting laser (VCSEL) is a solid-state semiconductor laser in which light is emitted from the surface of a monolithic structure of semiconductor layers, in a direction normal to the surface. This is in contrast to the more commonly used edge-emitting laser, in which light is emitted from the edge of the wafer. Whereas edge-emitting lasers rely on facet mirrors formed at the wafer edge by cleaving or dry etching, the operation of VCSELs is enabled through the use of distributed Bragg reflector (DBR) mirrors for longitudinal optical confinement. VCSELs are advantageous over edge-emitting lasers in that (i) they have a lower-divergence, circularly-shaped laser beam, (ii) may be manufactured using standard fabrication processes such as those used in silicon VLSI technology, (iii) may be tested at the wafer level prior to packaging, and (iv) may be fabricated in dense 2-dimensional arrays for lower cost and higher volume.
As in any laser, the overall structure of a VCSEL is that of two end mirrors on each side of an active region, the active region producing the light responsive to an electric current therethrough. However, the active region is a thin semiconductor structure, and the end mirrors are distributed Bragg reflector mirrors (xe2x80x9cDBR mirrorsxe2x80x9d) comprising alternating layers of differently-indexed material such that light of only the desired wavelengths is reflected. Further general information on VCSELs may be found in the following references, each of which is incorporated by reference herein: Cheng and Dutta, eds., Vertical-Cavity Surface-Emitting Lasers: Technology and Applications, Vol. 10 of Optoelectronic Properties of Semiconductors and Superlattices, Manasreh, ed., Gordon and Breach Science Publishers (2000); Sale, T. E., Vertical Cavity Surface Emitting Lasers, Wiley and Sons (1995); and Dutton, Understanding Optical Communications (Prentice Hall 1998), at pp. 159-161.
Today, VCSELs are widely used in local area networks and in very short-reach applications such as connections between electronic routers, the back planes of computers, computers to disk farms and computers to sensors, and curb to home, at wavelengths of about 650 nm to 980 nm. For long-distance fiber optic communications applications, e.g. generally requiring transmission distances of greater than 10 km, VCSELS having longer output wavelengths of 1300-1550 nm will be useful. Today, commercially-available VCSELs are directly modulated to frequencies of 2.5 Gb/s, while direct modulation of up to 20 Gb/s has been reached in research laboratories. With increasing demands for bandwidth, it becomes desirable to develop practical and reliable VCSELs with modulation frequencies of greater than 10 Gb/s.
One problem in the practical realization of higher VCSEL modulation frequencies exists in regard to current confinement, which relates generally to the confinement of electrical current passing through the active layer to a small lateral portion of that layer to increase current density. At higher modulation frequencies, it is necessary to increase the photon density in the optical cavity to increase the relaxation oscillation frequency of the laser. The increase in photon density necessitates an increase in the injected current density of the laser which, in turn, increases heating at the active layer. Depending on the laser structure, this increase in heat can cause the laser to have a reduced lifetime.
Proper current confinement of the injected electrons and holes to the active area is important to achieve a highly efficient laser. It is not uncommon for a VCSEL to have a desired xe2x80x9cwall plugxe2x80x9d efficiency, i.e., the ratio of optical power emitted by the laser over the electrical power applied, in excess of 50%. High wall plug efficiency is necessary for high frequency operation to minimize excess heating. Several methods are in use to achieve proper current confinement, including proton implantation methods, mesa methods, buried regrowth methods, and oxide-based methods. Disadvantages exist with these methods, however. For example, one problem with proton implantation is that it cannot define a small sharp region due to the nature of ion implantation. As another example, in the mesa method, the mesa is non-planar and has difficulty in passivation, resulting in poor reliability. As another example, buried regrowth structures are more complex to manufacture and result in a reduced yield.
Oxide confinement, in which layers adjacent to the active layer are selectively oxidized such that current is confined to a small non-oxidized portion, is generally the most widely used method to manufacture low threshold and efficient VCSELs. See Jewell et. al., xe2x80x9cVertical cavity surface emitting lasers: design, growth, fabrication, characterizationxe2x80x9d, IEEE Journal of Quantum Electronics, vol. 27, no. 6, pp. 1332-1346 (June 1991), which is incorporated by reference herein. Oxide-confined VCSELs have achieved sub-milliampere threshold current and single mode operation. See, e.g., Deppe et al, xe2x80x9cLow-threshold vertical cavity surface emitting lasers based on oxide confinement and high contrast distributed Bragg reflectors,xe2x80x9d IEEE Journal of Selected Topics in Quantum Electronics, vol. 3, no. 3, pp. 893-904 (June 1997); Nishiyama et. al., xe2x80x9cMulti-oxide layer structure for single mode operation in vertical cavity surface emitting lasers,xe2x80x9d IEEE Photonics Technology Letters, vol. 12, no. 6, pp. 606-8 (June 2000); and Deppe, xe2x80x9cOptoelectronic Properties of Semiconductors and Superlattices,xe2x80x9d at Chapter 1 of Cheng and Dutta, supra, each of these references being hereby incorporated by reference herein.
Oxide confinement methods have some characteristics that may result in reduced lifetime of the laser at high current injection operations, as is needed for high modulation frequencies. For example, according to one oxide confinement method discussed in Choquette, xe2x80x9cThe Technology of Selectively Oxidized Vertical Cavity Lasers,xe2x80x9d at Chapter 2 of Cheng and Dutta, supra, which is incorporated by reference herein, the high content of Al in an AlGaAs or AlAs layer is oxidized using nitrogen and steam at a temperature of 420-450 degrees Celsius. However, the AlGaAs or AlAs layer undergoes a dimensional change after the oxidation. This dimensional change causes stresses in the current confinement area, as well as in the active area, which is in close proximity to the current confinement area. These stresses, as well as the method of fabricating the oxide confining layer for the VCSEL, are discussed in Choquette, supra.
Accordingly, it would be desirable to provide a VCSEL having a current confinement structure with advantages similar to those of the oxide confinement method, while at the same time avoiding the disadvantages of the material stresses caused by the oxide confinement method that may reduce the lifetime of the VCSEL device.
According to a preferred embodiment, a VCSEL structure and fabrication method therefor are provided, wherein a subsurface air, gas, or vacuum current confinement method is used to restrict the area of electrical flow in the active region. Because air, gas, or vacuum is used, the stresses caused in oxidation-based current confinement methods are avoided. Additionally, because the confinement is achieved by subsurface cavity structures, overall mechanical strength of the current-confining region is maintained, such that possible xe2x80x9ccollapsexe2x80x9d of the current-confining structure is not a problem.
According to a preferred embodiment, a vertical cavity structure is formed comprising a lower distributed Bragg reflector (DBR), an upper DBR, an active layer positioned between the lower DBR and the upper DBR, and a current confinement layer positioned adjacent to the active layer. The current confinement layer comprises a semiconductor material that is laterally etchable by a selective etchant to which the upper DBR material and active layer materials are etch-resistant. Three or more hollow vertical shafts are formed through the upper DBR layer and current confinement layer, the hollow vertical shafts being positioned outside a desired current confinement zone, the centers of the hollow vertical shafts forming a polygon that laterally circumscribes an area around the desired current confinement zone. The selective etchant is then applied, laterally etching away the current confinement layer outwardly from the axis of each hollow vertical shaft, forming a subsurface cavity around each hollow vertical shaft at the current confinement layer. The lateral etching process is continued until the subsurface cavities merge together to form a single subsurface circumferential cavity around the desired current confinement zone. Electrical current is confined to the current confinement zone as it passes through the current confinement layer. After optionally filling the subsurface circumferential cavity with an inert gas or other inert non-solid material, the hollow vertical shafts are sealed off by a polyimide material, a gold plating material, and/or a dielectric material.